But Saskatchewan says the federal government is intruding on provincial authority simply because it doesn’t like provincial policies.

“This Big-Brother-Ottawa-knows-best is inimical to federalism,” lawyer Mitch McAdam told the five-justice panel. “You don’t need to throw out the Constitution in this case in order to save the planet.”

The federal Greenhouse Gas Pollution Pricing Act, which levies a charge on gasoline, other fossil and on industrial polluters, kicked in on April 1. While Ottawa calls the levy — currently four cents a litre on gasoline — a regulatory charge designed to change behaviour, Ontario and Saskatchewan call it an illegal tax.

“Consumers are simply told to do one thing: Pay more tax,” McAdam said. “The power to tax is the power to destroy.”

Earlier, a lawyer for British Columbia, which sides with Ottawa, said Canadians elected the federal government to address climate change. The law, he said, doesn’t get in the way of provinces’ ability to do their part.